Hitherto, an electrophotographic image forming apparatus such as a digital copying machine, a laser beam printer, or a facsimile apparatus includes a light scanning apparatus. The light scanning apparatus scans a surface of an electrophotographic photosensitive drum (hereinafter referred to as “photosensitive member”) with a laser beam (hereinafter referred to as “light beam”) modulated based on the image information, to thereby form an electrostatic latent image thereon. The electrostatic latent image is developed by a developing device into a toner image with toner. The toner image on the photosensitive member is transferred onto a recording medium by a transfer device. A fixing device fixes the toner image transferred onto the recording medium, to thereby form an image on the recording medium.
The light scanning apparatus includes a light source configured to emit a light beam and a rotary polygon mirror configured to deflect the light beam emitted from the light source so that the light beam scans a photosensitive member. The light scanning apparatus generates a synchronizing signal (hereinafter referred to as “BD signal”) in a main-scanning direction in order to maintain the same image writing start position (scan starting position) on the photosensitive member during each scan. When the light beam enters a beam detector (hereinafter referred to as “BD”), the BD signal is output from the BD.
The light scanning apparatus starts to rotate the rotary polygon mirror before image formation, but it is unknown which timing the light beam is to be emitted at so that the light beam enters the BD. Therefore, in a state in which the rotary polygon mirror is stably rotated at a main rotation speed for image formation, the light source is kept on (laser full lighting-up) over a predetermined period of time. Then, the timing at which the light beam enters the BD is detected from the BD signal, and a timing to turn on the light source so that the light beam enters the BD is determined. This is referred to as “BD search (BD timing search)”. When the BD search is completed, the image forming apparatus becomes ready to start the image formation.
In the BD search, the light source is kept on until at least the first BD signal is detected, and hence the photosensitive member is exposed to the light beam emitted from the light source during the time. The exposure of the photosensitive member in the BD search causes marking back to occur on the recording medium. Therefore, there is proposed a light scanning apparatus including a mark provided on a rotor configured to hold the rotary polygon mirror, and an optical sensor configured to irradiate the mark with light, detect light reflected therefrom, and output a position signal indicating a rotational position of the rotor (PTL 1). During pre-rotation processing at a time of warm-up before an image forming operation, the rotary polygon mirror is rotated at the main rotation speed, and a time difference between the position signal and the BD signal is obtained and stored in a memory. After that, the light source is turned off. By turning on the light source based on the time difference read from the memory immediately before the image formation, it is possible to determine a lighting-up timing of the light source for obtaining the BD signal without exposing the photosensitive member. Therefore, it is possible to prevent the marking back from occurring on the recording medium.